The vast agora of Kos seems to have preserved its ancient configuration still in the 4thc.; the datum is attested by continuous restorations of the eastern portico (Fig. I.1.3.1), damaged by some seismic event. This aspect is particularly remarkable because confirms the long duration of the porticoes and the square itself. Only after the Arab invasions of the mid-seventh c., the city seems to contract, with the big square gradually abandoned and converted to agricultural zone. Indications in this regard comes from the exploration of the central sector, where in recent years the Greek Archaeological Service and the Archaeological Institute of Aegean Studies have found materials that confirm the long period of frequentation of the agora as a public area.
A phase of transformation must have occurred after the earthquake of the second half of the 5thc., which involved a strong elevation in the levels of frequentation, now at 4.16 m circa above sea level, whilst the levels in the Hellenistic period were at or near 3:20 m above sea level. The same altitude of 4.16 m would have also obliterated the krepis of the stoai of the agora, surpassing the medium level of the stylobate, equal to ca. 4:00 m above sea level. In this phase a large building, probably public, was built in the southwest corner of the square (Figs. I.1.3.2-3), using materials taken from marble monuments of Hellenistic-Roman period now destroyed.
Across the plateia-decumanus, the great Southern Stoa which since the 3rdc. B.C. had constituted the limit on this side of the agora, was destroyed. From the 4thc., it seems that the central part of the Stoa had been already demolished to make way for an advance toward north of the Central Baths, now renovated. Later, the western sector of the Stoa, in front of the so-called ‘Casa Romana’, presents a reusing phase, demonstrated by the construction of several rooms, built in the space once occupied by the rear compartments of the Stoa and raised on its residual structures (Figs. I.1.3.4-5). Their scheme, albeit at a reduced size, reproduces the original layout, defined in a large vestibule which leads to two smaller inner compartments. At the moment the function remains uncertain, although it has been suggested a possible continuity of the public role of the Stoa itself, which was undoubtedly linked to the complex requirements of the polis civic life. In the central sector of the agora, at the current state of research, it is unclear whether the tholos built in imperial times (cfr. Fig. I.1.1.1) was still visible, or, more likely, had already been destroyed by the earthquakes of 5th- 6thc. and then despoiled. Recent excavations in the area have nevertheless highlighted sewers and drainage, continually rebuilt up to a late age, proof of a long usage of the place as a public area.
At the north end of the agora, as early as the 2ndc., the northern walls had been largely demolished, to build a monumental front on the harbour (cfr. § I.1.1). The complex must have lived a long time, as attested by subsequent layers of frequentation and the artisan installations built in the rear rooms of the Eastern Stoa. Even within the vaulted passages of the northern front traces of occupation are visible, reflecting the fact that their collapse must have occurred fairly late, maybe because of the earthquake of 554, according to Morricone. Other installations for the production of glass objects, used until the mid-sixth c., were brought to light near the south-east sector of the agora.
In the Early Byzantine city, at the port, restored again after the devastating earthquake of mid-sixth c., the commercial square near the dock, the old emporium, must have been still frequented. The mercantile vocation of the square must also have remained for long if right here the medieval Porta tou Forou, the western access to the fortification of the chora, and the church of Panagia tou Forou arose. Given their location (Fig. I.1.3.8), the Forum to which these toponyms refer, more than in the civil agora - that must have retained official and representative functions until it was progressively abandoned - must be rather recognized in the commercial square in the harbour district. The existence of a commercial agora and a port still in regular use in the 5th- 6thc. is an interesting point, because it shows that in Late Antiquity one of the principal factors of the economic growth of Kos derived from the activity of its maritime trade, ensured by the position on an important commercial route, the annona route between Alexandria and Constantinople.